Apatite inclusions in zircon reveal a crustal origin for HREE depletion in Archean granitoids

Clémentine Antoine , Lili Loth , Joaquin Bastias-Silva, Martin Guitreau, Nicolas David Greber, Luca Caricchi 

Reconstructing the geodynamic conditions of the Hadean and Archean is essential for understanding Earth’s evolution and the conditions that ultimately enabled life to emerge. Archean tonalite–trondhjemite–granodiorite (TTG) suites have been studied extensively because their evolved compositions provide clues to the processes that formed the early continental crust. Their trace-element signatures, particularly heavy rare earth element (HREE) depletion, have traditionally been attributed to melting of hydrous basaltic crust under pressure-temperature conditions within the garnet stability field. However, TTG compositional diversity may also arise during magmatic differentiation within the crust, rather than from differences in melting conditions alone. Distinguishing these mechanisms requires constraints on both primitive melt compositions and their evolution during differentiation. Whole-rock analyses provide only snapshots of this evolution and may be modified by alteration. Here, we use apatite inclusions in zircon to reconstruct the chemical evolution of TTG and related granitoid magmas. We first present a four-criterion protocol for distinguishing primary from secondary apatite inclusions and apply it to inclusions from five cratons, identifying 92 of 153 analysed grains as primary. Melt compositions reconstructed from these primary apatites show little or no HREE depletion in many samples, whereas HREE depletion increases with the inferred degree of melt fractionation. We therefore propose that the HREE-depleted signature of many TTGs can develop through amphibole-bearing crystallization in the crust, without requiring garnet in the source in every case. These findings have implications for models of early continental-crust formation and Archean geodynamics.